In the past, various friction stir welding apparatuses for overlapped joints have been proposed as apparatuses for overlapping and joining two members to be joined (for example, refer to JP2002-035962 and JP2002-035964).
FIG. 8 shows an example of a conventional friction stir welding apparatus for overlapped joint. As shown in FIG. 8, the friction stir welding apparatus 21 for overlapped joints joins a pair of members to be joined 22, 23 that are caused to overlap. The apparatus 21 comprises a rotor 24 that has a shoulder 25, and a cylindrical probe 26 provided so as to protrude from the shoulder 25. The probe 26 is formed with a diameter that is smaller than that of the rotor 24, and is disposed concentrically with the center axis C of the rotor 24.
When the pair of members 22, 23 are overlap joined, while rotating the rotor 24 in the direction of the arrow R about the center axis C of the rotor 24, the friction stir welding apparatus 21 for overlapped joints is pressed up against the surface of the upper member to be joined 22 of the overlapped members 22, 23 and caused to move along a prescribed joining line (not illustrated). When this is done, the members to be joined 22, 23 in the region surrounding the probe soften, and the softened members 22, 23 are stirred and flow plastically. The result is the formation of a joint 27 along the prescribed joining line.
FIG. 9 is a photomicrograph of a joint formed using a conventional friction stir welding apparatus for overlapped joints. As shown in FIG. 9(b) through (d), in a conventional friction stir welding apparatus for overlapped joints 21, there is the problem that it is difficult for the softened members to be joined 22, 23 in the region surrounding the probe 26 to flow upward and downward, making it easy for void defects to occur in the region surrounding the tip 26a of the probe 26.
Also, in the prior art, a friction stir welding apparatus for overlapped joints was proposed for the purpose of solving this problem of void defects.
FIG. 10 shows another example of a conventional friction stir welding apparatus for overlapped joints. In the friction stir welding apparatus 31 for overlapped joints as shown in FIG. 10, a helical groove 37 is formed in the peripheral surface of the probe 36, from the tip 36a of the probe 36 to the shoulder 35 of the rotor 34.
In a friction stir welding apparatus for overlapped joints such as this, as shown in FIG. 10, the softened members to be joined 32, 33 in the region surrounding the probe 36 flow upward and downward because of the helical groove 37, thereby enabling prevention of void defects in the region surrounding the tip 36a of the probe 36.
In the above-described constitution, however, the upward and downward flow of the members to be joined 32, 33 in the region near the boundary face 38 between the members to be joined 32, 33 increases, so that the softened members to be joined flow so as to be curled upward after flowing downward, as shown in FIG. 10. As a result, as shown in FIG. 11, an oxide film at the boundary face 38 between the members to be joined 32, 33 remains in a condition of being curled upward.
When an oxide film, which is a location having a weak cohesive force, remains in an upwardly curled condition, as shown in FIG. 12, the actual plate thickness t1 of the upper members 32 becomes thinner than the original plate thickness t0 thereof, thereby resulting in a reduction of the strength of the joint in the members to be joined 32, 33. For example, in a case such as shown in FIG. 12, in which mutually opposing forces act which are parallel to the boundary face 38 between the members 32, 33 (in the tensile shearing direction), there is the problem of a tendency for a crack to occur in the residual oxide film 39.